1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for calibrating a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system, i.e. the bottom of the projection system, and the substrate. This enables more accurate projections and imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the immersion liquid may be regarded as increasing the effective numerical aperture number NA of the system and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein. Thus, a lithographic apparatus may be provided with a fluid provider (also referred to as an immersion head) which is arranged to provide the immersion liquid, or to keep the liquid in its place. The liquid may be flowing to avoid local heating.
The substrate, or the substrate and the substrate table, may be immersed in a bath of immersion liquid. An example of such an arrangement is disclosed in U.S. Pat. No. 4,509,852 which hereby is incorporated by reference in its entirety. Alternatively, the immersion liquid may be provided by a liquid supply system only on a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system, the substrate generally having a larger surface area than the final element of the projection system. An example of such an arrangement is disclosed in International Patent Application No. 99/49,504 which hereby is incorporated by reference in its entirety. The liquid is supplied by at least one inlet on the substrate, preferably along a direction of movement of the substrate relative to the final element of the projection system, and the liquid is discharged by at least one outlet which may be connected to a low pressure source. Various orientations and numbers of inlets and outlets positioned near the periphery of the final element are possible. Further, a liquid supply system may be provided with a seal member which extends along at least a part of a boundary of a space between the final element of the projection system and the substrate table. The seal member is substantially stationary relative to the projection system in the XY plane of an XYZ system of coordinates, though there may be some relative movement in the Z direction (the direction of the optical axis of the projection system). A seal is formed between the seal member and the surface of the substrate. The seal may be a contactless seal such as a gas seal, which may further function as a gas bearing. An example of such an arrangement is disclosed in European Patent Application No. 03252955.4, which hereby is incorporated by reference in its entirety.
European Patent Application No. 03257072.3, which hereby is incorporated by reference in its entirety, discloses a twin or dual stage immersion lithography apparatus. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without the presence of an immersion liquid, and exposure is carried out with a stage at a second position, where an immersion liquid is present. Alternatively, the apparatus has only one stage.
In lithographic apparatuses of the types described above, an interferometer system may be used for measuring the position and displacement of an object, such as a substrate or a mask, in a plane which is parallel to the XY plane, by X and Y measuring mirrors which are arranged on a holder or object table, such as a substrate table or a mask table, carrying the object. The system includes a generator which generates a plurality of measuring beams and directs the measuring beams along a plurality of measuring axes which are parallel to the XY plane and extend to and from the measuring mirrors, and radiation-sensitive detectors which convert measuring beams reflected by measuring mirrors into electric measuring signals which include information about the X and Y position of the object. The number of X and Y measuring axes is at least equal to the number of object movements to be measured interferometrically. Further, Z measuring mirrors are arranged on the holder of the object at an angle to the XY plane, to which end the interferometer system has a Z measuring axis and is provided with a generator which generates a Z measuring beam and directs the measuring beam onto the Z measuring mirror, and a Z detector for converting the Z measuring beam from the Z measuring mirror into an electric measuring signal which comprises information about the Z position of the object. The lithographic apparatus may further include a metrology frame supporting Z reference mirrors essentially extending in the XY plane. Alternatively, such Z reference mirrors may be secured to a holder of the projections lens system. Further details of the interferometer system for determining the X, Y and Z positions of an object holder of an object are disclosed in U.S. Pat. No. 6,020,964 which hereby is incorporated by reference in its entirety.
A calibration of the interferometer system for the Z direction includes a calibration of the linear Z dependency on X, also referred to as the sensitivity pzx. Conventionally, this calibration is performed by projecting an image in air between the projection system and the substrate, while adapting the Z position with a substrate table positioning system to obtain a measuring signal at two different positions, as viewed in the X direction. As an example, a sensor to be used for this calibration may be a Transmission Image Sensor (TIS) measuring through the projection system. The TIS enables a fine calibration of the Z interferometer, provided that the substrate table is located within a predetermined Z range, which is obtained in a coarse calibration. The TIS will have been adjusted to measuring in a wet condition, i.e. in the presence of an immersion liquid between the final element of the projection system and the substrate.
Typically, one of the Z reference mirrors may not be aligned to the other, and/or one or both of the Z reference mirrors may not extend at right angles to the Z direction. A reason for such an offset may be initial alignment (in)accuracy, initial adjustment (in)accuracy, inadvertent touching, shock or collision of the Z reference mirrors or the metrology frame on which they may be mounted, e.g. during maintenance. As a result, in the X dependency of Z, which may be expressed asZ=pzx*X+ . . . (where pzx, a parameter for the linear X dependency, may be seen as indicating the average offset angle of the Z reference mirrors), the value of pzx is uncalibrated. In an immersion type lithographic apparatus, unacceptable Z error may occur when the measuring system is influenced by the offset Z reference mirrors and pzx is uncalibrated. This may cause unexpected widening or narrowing of the layer of immersion liquid. On the other hand, a calibration of pzx can only be performed in a wet condition, e.g. using the TIS. Thus, on the one hand pzx can only be calibrated by going to the wet condition, but on the other hand pzx must be calibrated at least coarsely before going to the wet condition.